1. Field of the Invention
This invention relates to lock-up control devices that control the engaging force of lock-up clutches, which share engine outputs with torque converters to transmit engine output toward input shafts of transmissions of cars. This application is based on Patent Application No. Hei 9-252534 and Patent Application No. 9-257224 both filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Japanese Patent Application, Publication No. Hei 7-332479 discloses an example of a lock-up control device. This lock-up control device has a map of target slip ratios, which are considered effective to avoid occurrence of abnormal sounds such as indistinct sounds and abnormal vibrations such as surging. Herein, the target slip ratios are created in advance by experiments regarding real car travel profiles, wherein they are produced in connection with car speeds and accelerator pedal openings. Thus, the lock-up control device controls the lock-up clutch to provide the target slip, which is read from the map in connection with the car speed and accelerator pedal opening.
As described above, the map for controlling the lock-up clutch is created based on experimental data. So, considerable amounts of labor and cost are required to create such a map.
Even in the same type of the car, different maps are required in response to changes of gear ratios. So, every time the gear ratio is changed, it is necessary to provide a specific map whose content is created through experiments. As a result, enormous amounts of labor and cost are required to create multiple maps with regard to the same type of the car.
At a shift-up event, the aforementioned lock-up control device unconditionally turns off the lock-up clutch so as to set the engaging force at zero, so that only the torque converter works to transmit the engine output toward the input shaft.
In consideration of the improvement of fuel efficiency, it is preferable that the lock-up clutch is set at a "tight" side, in other words, the lock-up clutch should be set as tightly as possible.
For this reason, even if the lock-up clutch is set at the tight side before a gear change event, a gear change is performed while the lock-up clutch is maintained at the tight side.
However, in the case of the gearshift which is performed while the lock-up clutch is maintained at the tight side, a shift-up operation reduces a number of revolutions of the input shaft of the transmission, so engine speed is reduced as well. In such a case, it is predicted that the engine speed is reduced to a critical one that will easily induce occurrence of abnormal sounds such as indistinct sounds and abnormal vibrations such as surging.